1. Field of the Invention
The invention relates to a method of fabricating a field emission cold cathode, and more particularly to a method of fabricating a field emission cold cathode capable of reducing a divergence angle of emitted electron beams.
2. Description of the Related Art
A field emission cold cathode attracts attention as a new electron source substituted for a hot cathode utilizing thermionic emission. A field emission cold cathode is provided with a so-called emitter electrode having a sharpened tip, and emits a mass of electrons when a high intensity field, specifically in the range of 2.times.10.sup.7 V/cm to 5.times.10.sup.7 V/cm or greater, is produced around the sharpened tip of the emitter electrode. Accordingly, performance of a device is greatly dependent on sharpness of the tip. It is said that a point of an emitter electrode is required to have a radius of curvature equal to or below hundreds of angstroms.
In order to produce an electric field, it is necessary that emitter electrodes are disposed with spacing between adjacent ones being about 1 .mu.m or smaller, and that a voltage in the range of tens of to hundreds of volts is applied to emitter electrodes. In an actually used product, emitter electrodes in the range of thousands to tens of thousands in number are disposed on a common substrate.
Thus, a field emission cold cathode is fabricated in general by means of fine processing technology widely used in the semiconductor manufacturing field. A field emission cold cathode is applied to an electron tube such as a flat panel display, a micro vacuum tube, a micro-wave tube and a cathode ray tube (CRT), and an electron source for various sensors.
One of field emission cold cathodes is a so-called Spindt type field emission cold cathode, a perspective view of which is illustrated in FIG. 1. The illustrated Spindt type field emission cold cathode includes an electrically conductive substrate 51, a plurality of cone-shaped emitter electrodes 56 made of electrically conductive material and formed on the substrate 51, an insulating layer 52 formed with a plurality of cavities and formed on the substrate 51, and a gate electrode 53 formed with a plurality of openings each of which surrounds the emitter electrode 56.
As illustrated in FIG. 1, electron beams 59 emitted from the emitter electrodes 56 are divergent to some degree about a perpendicularly extending axis of the emitter electrodes 56. If each of the electron beams 59 emitted from each of the emitter electrodes 56 has divergence to a greater degree, all of the electron beams 59 emitted from an emitter array have greater divergence accordingly. For instance, when the illustrated emitter array is used for a flat panel display, the divergence of the electron beams 59 would cause excitation of fluorescent material by a picture element located adjacent, resulting in deterioration of crosstalk.
Japanese Unexamined Patent Publication No. 7-122179 has suggested a field emission cold cathode formed with a focusing electrode in order to depress divergence of electron beams. As illustrated in FIG. 2I, the suggested field emission cold cathode includes a substrate 61 including a substrate 61 consisting of a glass substrate 71, an electrically conductive layer 72 formed on the glass substrate 71 and a resistive layer 73 formed on the layer 72, a plurality of conical emitter electrodes 66 formed on the substrate 61, a first insulating layer 62 formed on the resistive layer 73, a gate electrode 63 formed on the first insulating layer 62 and formed with an opening surrounding a point of the emitter electrode 66, a second insulating layer 64 formed on the gate electrode layer 63, and a focusing electrode 65 formed on the second insulating layer 64 formed with an opening in alignment with the opening formed with the gate electrode 63. A lower voltage than a voltage to be applied to the gate electrode 63 is applied to the focusing electrode 65 to thereby converge electron beams emitted from the emitter electrodes 66.
A method of fabricating the above mentioned field emission cold cathode is explained hereinbelow with reference to FIGS. 2A to 2I.
First, as illustrated in FIG. 2A, the electrically conductive layer 72 and the resistive layer 73 are deposited on the glass substrate 71. Then, a silicon dioxide (SiO.sub.2) film as the first insulating layer 62 and a niobium (Nb) film as the gate electrode 63 are formed on the resistive layer 73.
Then, as illustrated in FIG. 2B, an aluminum layer as a mask layer 68 is deposited over the gate electrode 63. Then, a first resist layer 75 patterned by photolithography is formed on the mask layer 68, as illustrated in FIG. 2C. The mask layer 68 is etched with the first resist layer 75 being used as a mask to thereby form a ring-shaped mask layer 69, as illustrated in FIG. 2D.
Then, the second insulating layer 64 and the focusing electrode 65 are deposited over a resultant, as illustrated in FIG. 2E.
Then, a second resist layer 76 is formed over a resultant, and is patterned by lithography so that the layer 76 has an opening having a diameter equal to an outer diameter S of the ring-shaped mask 69. Then, reactive ion etching (RIE) is carried out with the patterned second resist layer 76 being used as a mask to thereby etch the focusing electrode 65 and the second insulating layer 64. As a result, there is formed a first opening 78 in which the ring-shaped mask 69 and the gate electrode 63 appear, as illustrated in FIG. 2F.
Then, the niobium film or the gate electrode 63 is dry-etched with SF.sub.6 and the silicon dioxide film or the first insulating layer 62 are dry-etched with CHF.sub.3 both with the ring-shaped mask 69 being used as a mask, to thereby form a second opening 79 in the gate electrode 63 and the first insulating layer 62, as illustrated in FIG. 2G.
Then, as illustrated in FIG. 2H, there is carried out oblique evaporation with the resultant being rotated, to thereby form a sacrifice layer 77 on the second resist layer 76 and further on an inner sidewall of the first opening 78 so that an opening area of the first opening 78 is almost equal to an opening area of the second opening 79. Herein, the sacrifice layer 77 is made of metal such as nickel (Ni) and aluminum (Al).
Then, molybdenum (Mo) is evaporated perpendicularly onto the resistive layer 73. Since molybdenum particles for deposition are masked by an opening 77a defined by the sacrifice layer 77 formed around the first opening 78, the molybdenum particles are deposited on the resistive layer 73 as if a shape of the opening 77a is projected onto the resistive layer 73. The molybdenum particles deposit also on the sacrifice layer 77 to thereby form a molybdenum layer 67. Hence, with the deposition of the molybdenum particles on the sacrifice layer 77, a diameter of the opening 77a of the sacrifice layer 77 is gradually decreased. Accordingly, a diameter of the deposition of the molybdenum particles on the resistive layer 73 is gradually decreased, resulting in that a conical emitter electrode 66 is formed, as illustrated in FIG. 2H.
Then, a resultant is soaked into phosphoric acid to thereby remove the molybdenum layer 67, the sacrifice layer 77 and the second resist layer 76. Thus, there is completed a field emission cold cathode 70 as illustrated in FIG. 2I.
As illustrated in FIG. 3, the ring-shaped mask 69 may be designed to have a greater outer diameter than an inner diameter of the first opening 78. According to the above mentioned Publication No. 7-122179, this structure brings an advantage that an accuracy in registration for the formation of the first opening 78 may be decreased.
One of field emission cold cathodes having no focusing electrode is suggested in Japanese Patent Application No. 7-60886, which does not constitute a prior art, but is described hereinbelow for better understanding of the present invention. As illustrated in FIG. 4A, the suggested field emission cold cathode includes a substrate 101, a first insulating layer 104 formed on the substrate 101, a second insulating layer 105 formed on the first insulating layer 104, a gate electrode 103 formed on the second insulating layer 105, and an emitter electrode 106 formed on the substrate 101. The illustrated cold cathode is characterized by double insulating layers formed with openings having different inner diameters.
The formation of the two insulating layers with openings having different inner diameters improves insulation performance between the substrate 101 and the gate electrode 103. In the field emission cold cathode illustrated in FIG. 4A, a diameter Dg of an opening formed with the gate electrode 103 is equal to a diameter Di of an opening formed with the second insulating layer 105. However, as illustrated in FIG. 4B, the diameter Dg may be designed to be greater than the diameter Di (Dg&gt;Di).
Japanese Unexamined Patent Publication No. 6-131970 has suggested a method of forming an emitter electrode including steps of forming two sacrifice layers. Hereinbelow is explained the suggested method.
As illustrated in FIG. 5A, an oxide film 82, a tungsten film 83 and a first sacrifice layer 91 are formed on a substrate 81. Then, a resist layer 89 is formed over the first sacrifice layer 91, and is patterned. Then, the first sacrifice layer 91 is formed with an opening by etching with the patterned resist layer 89 being used as a mask.
After the removal of the resist layer 89, a second sacrifice layer 92 is deposited all over a resultant, as illustrated in FIG. 5B. Then, the second sacrifice layer 92 is formed with an opening, and thereafter a cavity is formed in the tungsten layer 83 and the oxide layer 82. Then, molybdenum particles are evaporated onto the substrate 81 to thereby form a small emitter electrode 86, as illustrated, in FIG. 5C. At the same time, a molybdenum layer 86a is formed over the second sacrifice layer 92.
The second sacrifice layer 92 is etched in selective areas to thereby remove or lift-off the molybdenum layer 86a deposited thereon. Then, molybdenum particles are evaporated onto the small emitter electrode 86 with the first sacrifice layer 91 being used as a mask, to thereby make the emitter electrode 86 grow, as illustrated in FIG. 5D. Then, the first sacrifice layer 91 together with a molybdenum layer deposited on the first sacrifice layer 91 are etched for lift-off. Thus, there is completed a field emission cold cathode 90, as illustrated in FIG. 5E.
The above mentioned conventional field emission cold cathode including a focusing electrode described with reference to FIGS. 2A to 2I, suggested in Japanese Unexamined Patent Publication No. 7-122179, has problems as follows.
The first problem is that an emitter electrode is formed in inclination if it is to be formed near an outer edge of the substrate, and that an emitter electrode is formed in no alignment with an opening formed with a gate electrode. The reason is explained hereinbelow with reference to FIGS. 6A and 6B.
A substrate 1 and an evaporation source 2 are positioned as illustrated in FIG. 6A when a film is formed by vacuum evaporation process. Particles for deposition are emitted perpendicularly onto a central region of the substrate 1, namely particles are emitted with an incident angle perpendicular to the substrate, and particles for deposition are emitted with a smaller incident angle onto a region further away from the central region of the substrate. Particles for deposition are emitted onto an outer edge of the substrate 1 with an incident angle .theta..
FIG. 6B is a cross-sectional view of a portion near the outer edge of the substrate 1. As illustrated, the sacrifice layer 77 formed on the electrode layer 5 acts as a mask for the formation of the emitter electrode 6. However, the sacrifice layer 77 acting as a mask is located at a distance from the substrate on which the emitter electrode 6 is to be formed. In addition, the particles for deposition have an almost parallel laminar flow at the central region of the substrate 1, but arrive at portions near the outer edge of the substrate 1 with an incident angle .theta.. Hence, compared to a field emission cold cathode having no focusing electrode in which the emitter electrode is formed by using a sacrifice layer as a mask which sacrifice layer is formed with an opening and formed on the gate electrode, a summit of the emitter electrode is made eccentric to a greater degree to the opening formed with the gate electrode, and the emitter electrode is formed in greater inclination for the same incident angle .theta., because the sacrifice layer acting as a mask is located further away from the substrate on which the emitter electrode is to be formed.
The second problem is large dispersion in shape of emitter electrodes. According to the above mentioned Publication No. 7-122179, as illustrated in FIG. 2H, this is because a diameter of an opening of the focusing electrode 65 is designed to be 1.2-2.0 times greater than a diameter of an opening of the gate electrode 63, and the sacrifice layer 77 formed around an opening of the focusing electrode 65 is used as a mask for the formation of the emitter electrode 66. That is, in the conventional method, it is necessary to deposit a large amount of sacrifice layer material on the focusing electrode 65 in order to equalize a diameter of a large opening of the focusing electrode 65 to a diameter of a small opening of the gate electrode 63.
The sacrifice layer 77 is formed by oblique evaporation with the substrate 71 being rotated. An opening formed in the sacrifice layer 77 deposited on the second resist layer 76 is initially circular, however, as a thickness of the sacrifice layer 77 is increased, the opening includes much deformation in shape. Accordingly, such deformation in shape of the opening causes a shape of the emitter electrode 66 to be deformed, because the deformed shape of the opening is projected to a shape of the emitter electrode 66. Thus, large dispersion in shape is found in a plurality of emitter electrodes.
The third problem is low designability both in a diameter of an opening of the focusing electrode 65 and a distance from the gate electrode 63 to the focusing electrode 65 which distance is equal to a thickness of the second insulating layer 64. These are important factors for depressing the divergence of electron beams. The reasons for the above mentioned low designability are that if an opening of the focusing electrode 65 is designed to have a greater diameter, the sacrifice layer 77 has to have a greater thickness, resulting in difficulty in obtaining proper shape of an emitter electrode, and that if the second insulating layer 64 is designed to have a greater thickness, as mentioned with reference to the first problem, it would be difficult for all of the emitter electrodes 66, in particular, emitter electrodes located near an outer edge of the substrate 71, to have a common shape.
The fourth problem is that the emitter electrodes are formed in misalignment with an opening of the gate electrode all over the substrate. The reason is as follows. In the conventional method, openings of the gate electrode and the focusing electrode are positioned relative to each other by means of two photolithography steps, and hence it is not possible to completely avoid misalignment in photolithography. As a result, when an emitter electrode is formed in an opening of the gate electrode with the focusing electrode having an opening being used as a mask, the emitter electrode is formed in accordance with the misalignment.
The fifth problem is difficulty in selecting material of which the ringshaped mask 69 is made. In the embodiment described in the above mentioned Publication No. 7-122179, the ring-shaped mask 69 is made of aluminum, and phosphoric acid is used for lift-off. However, aluminum is etched by phosphoric acid during carrying out lift-off. If aluminum of which the ring-shaped mask 69 is made is etched, durability and/or reliability of a device is deteriorated in particular in a device where, as illustrated in FIG. 3, the ring-shaped mask 69 is designed to have a greater outer diameter than an inner diameter of an opening of the focusing electrode.
The sixth problem is deposition of emitter material onto an opening of the gate electrode. In the above mentioned Publication No. 7-122179, after an area of an opening of the sacrifice layer 77 formed on the focusing electrode 65 is almost equalized to that an area of an opening of the gate electrode 63, emitter material is deposited to thereby form the emitter electrode 66 on the substrate. However, particles of emitter material may deposit to an opening of the gate electrode 63 on which no sacrifice layer is formed, due to deformation in an opening, dispersion in shape in openings, and inaccuracy in an incident angle of evaporation particles as set forth in the first problem. Hence, some evaporation particles cannot be removed even by lift-off.
Similarly, in a field emission cold cathode illustrated in FIG. 4B which has no focusing electrode, but has two-layered insulating layers, emitter material may deposit to a projecting end portion of the second insulating layer 105, and cannot be removed even by lift-off.
In the conventional method suggested in the above mentioned Japanese Unexamined Patent Publication No. 6-131970, the first and second sacrifice layers 91 and 92 are deposited one on another, and thereafter openings are formed by etching in the first and second sacrifice layers 91 and 92, as illustrated in FIG. 5A. The first and second sacrifice layers 91 and 92 are used as a mask for the formation of the emitter electrode 86. The process suggested in the above mentioned Publication has to repeat evaporation of emitter material and lift-off twice. Hence, in order to accomplish the process, the second sacrifice layer 92 has to be selectively removable against the first sacrifice layer 91.
Thus, there have to be carried out two steps separately, one for removing the first sacrifice layer 91, and the other for removing the second insulating layer 92. This would take much time, and make the process more complicated. In addition, the second sacrifice layer 92 has to be made of different material from the first sacrifice layer 91, which would decrease designability and increase the fabrication costs.